It would seem that your computer has been infected by some form of malware.

I would advise finding a reputable anti-virus and firewall for your computer and letting it run a scan while disconnected from the internet. Personally I use a combination of ZoneAlarm Extreme Security and AVG Anti-virus. I also use a well known preventative software called Spybot Search & Destory which is helpful to protect you from visiting dangerous sites or being harassed by advertising.

If you have tried an anti-virus program and still have this issue please let us know.

~Ducklz

Sounds like Malware.

Install, update, and run these programs in this order. They are listed in order of efficacy.
(Not all programs detect the same Malware, so you may need to run them all to solve your problem.)
These programs are all free for personal use, but some have limited functionality in the "free mode" - but those are features you really don't need to find and remove the problem that you have.
Note: If your Malware infection is bad enough and you are mis-directed to URL's other than what is posted, you may have to use a different PC to download these programs and use a USB stick to transfer them to the afflicted PC.

Malwarebytes' Anti-Malware - http://www.malwarebytes.org/mbam.php
SuperAntispyware - http://www.superantispyware.com/
AdAware - http://www.lavasoftusa.com/software/adaware/
Spybot Search & Destroy - http://www.safer-networking.org/en/index.html
Windows Defender: Home Page - http://windows.microsoft.com/en-US/wi.../windows-defender
Also, if you have a search engine re-direct problem, see this:
http://deletemalware.blogspot.com/2010/02/remove-google-redirect-virus.html

If these don't find it or can't clear it, post in one of these forums for specialized malware removal help:
http://www.spywarewarrior.com/index.php
http://forum.aumha.org/
http://www.spywareinfoforum.com/
http://bleepingcomputer.com